Arya spends most of her story on the road trip from hell. Seriously. At least Arya gets to spend a lot of time hanging out with her friends—though she also gets some QT with her enemies and frenemies, too. Here are a couple of the more noteworthy ones:
Unknown to him, Gendry is Robert Baratheon's illegitimate son, and he was forced to join the Night's Watch to save him from Cersei's murderous rampage against all of Robert's illegitimate children. Gendry travels with Arya throughout her story, but he is happiest when he's blacksmithing at the forge, the troubles of the world leaving him alone (48.Arya.98).
A crass member of the Night's Watch, Yoren was originally supposed to take Ned Stark to the Wall to take the black—but instead, he ends up saving Arya from King's Landing. Although he owes her and her father no true allegiance, Yoren protects Arya and all those under his watch. He is killed by Ser Amory's men when he refuses to yield to the knight.
Jaqen is an assassin who was being brought to the Wall by Yoren. Arya helps Rorge, Biter, and him escape a burning wagon during Amory's assault, and Jaqen promises her three deaths to pay the debt for the three lives. After he pays his debt, the assassin gives Arya a coin and instructions on how to find him again should she need him. Jaqen H'ghar is not his true name.
Both are criminals Yoren took from the King's Landing dungeon to join the Night's Watch. Rorge is missing his nose, and Biter, well, he bites. Both are unscrupulous characters who join the Brave Companions, and the only person they cooperate with is Jaqen H'ghar, likely because they fear him.
Originally, Hot Pie bullies Arya on their trip north, but the two eventually bond during their hardships. A coward, Hot Pie seems to come into his own as a baker in Harrenhal, or at least, he likes baking better than surviving by eating worms (48.Arya.11). He escapes with Arya once the castle is under Bolton rule.
Lommy Greenhands got his name because his arms have been dyed permanently green thanks to his work as a dyer's apprentice. During the battle with Ser Amory, Lommy was injured and unable to walk, and because of this, he curses Yoren for not yielding (20.Arya.30). He tries to yield to the Mountain men, but he is killed because none of the knights want to carry him.
Weasel is a starving girl found by Yoren's band on their trip north. After witnessing her mother's death and so many horrors, she has become mute and sticks close to Arya. When Arya's group is captured by the Mountain's men, Weasel escapes into the forest and is not seen again.